1. Field of the Invention
This invention relates to a pressurised-fluid steering gear for a large ship.
In recent years there has been an increasing recognition of the need for a more reliable steering gear for large ships, which has been underlined by at least one disaster to a supertanker attributable to a failure of its steering and is recognised by Protocols and Regulations made under the International Convention for the Safety of Life at Sea, calling as recently as 1980 for improvements in the safety and performance of the hydraulic steering systems of tankers in excess of 10,000 tons displacement and for all other types of ships.
2. Description of the Prior Art
In the known system illustrated in FIG. 1 of the accompanying drawings four hydraulic rams 4C,4D,4E and 4F rotate a rudder stock 1 to any desired position by moving a tiller arm 2 using a mechanism known as a Rapsons slide or variations of this mechanism. The hydraulic power is supplied by two hydraulic pumps 5A and 5B usually known as Helleshaw or Janney pumps, which have variable rate of flow and direction of flow and are driven by electric motors 5A' and 5B'. The direction and rate of oil flow is controlled by the position of a control rod 13 which is attached to the middle of a free-floating lever 12 known a a "hunting lever." One end of this lever is attached to a remote control piston 11 which is operated from the navigation bridge of the ship by remote control means (not shown). The other end of the lever 12 is attached through a link 12' to a point 12" on the radius of the rudder stock 1.
If the remote control lever 11 is moved to the right as viewed in FIG. 1, this will result in the lever 12 pivoting about its lower end and cause movement of the pump control rod 13 to the right also. This will start oil flow from the cylinders of rams 4C and 4F and into the cylinders 4D and 4E and result in anti-clockwise rotation of the rudder. This will cause the lower end of lever 12 attached to the rudder stock to move to the left. This movement results in lever 12 returning to its mid position and control rod 13 moving left until it reaches its original neutral position. Oil flow then ceases and rudder movement stops. It can thus be seen that the angle taken up by the rudder is directly dependent on the position and movement of remote control piston 11. In case of failure of the remote control system, a local hand steering system 10 may be connected up and used.
Other equipment in the system includes:
(a) Oil feed pumps 6A an 6B which are driven by the same electric motors driving the hydraulic pumps 5A and 5B and supply low pressure oil under constant pressure through non-return feed check valves 14 to both hydraulic systems thus ensuring that the oil is under a minimum pressure and topping up any oil lost from leakage. PA1 (b) Automatic isolating valves 5C and 5D which function to shut off and isolate one hydraulic pump when not in use and prevent back flow of oil between a working pump and an idle pump. PA1 (c) Manual isolating valves 8 serving to shut off two rams out of the four should this become necessary due to partial failure of the hydraulic or mechanical system. PA1 (d) Relief valves 7F which allow excess oil delivered from feed pumps 6 to flow back to an oil storage tank and keep the system under pressure, and PA1 (e) Relief valves 7C which will allow exchange of oil between the cylinders in case of high abnormal pressure which may result from action of rough seas on the rudder or by collision of the rudder with an external object.
It may be seen that should such a movement occur, this will cause movement of the rudder stock 1 and the lower point of lever 12 will be displaced. This causes the rod 13 to be displaced resulting in oil flow which will restore the rudder to its former position.
When both hydraulic pumps 5A and 5B are in operation, the rate of oil flow as determined by the position of the pump control rod 13 is double that when only one of the pumps 5A or 5B is in operation. This results in a quicker movement of the rudder with two pumps in use and is normally the case when the ship is manoeuvering in close waters such as when entering or leaving harbour. In the open sea quick rudder movements are not necessary and one pump is shut off.